1) Field of the Invention
This invention relates to an optical modulator module and an optical modulator suitable for use with an optical communication system.
2) Description of the Related Art
Conventionally, in optical communication systems, in order to modulate a light signal to be transmitted, an optical waveguide device in which an electro-optical crystal substrate such as a LiNbO3 or LiTaO2 substrate is used is utilized. The optical waveguide device is formed by forming a metal film partly on a crystal substrate and thermally diffusing the metal film into the crystal substrate or performing proton exchange in benzoic acid after patterning of the metal film to form an optical waveguide and then forming an electrode in the proximity of the optical waveguide.
FIGS. 14(a) and 14(b) are schematic views showing a conventional Mach-Zehnder type optical modulator 106 formed using Z-cut LiNbO3, and particularly, FIG. 14(a) is a top plan view of the Mach-Zehnder type optical modulator 106 and FIG. 14(b) is a sectional view taken along line A-A′ of FIG. 14(a). The optical modulator 106 shown in FIGS. 14(a) and 14(b) includes a LiNbO3 substrate 101, an optical waveguide 102, a signal electrode 103 and a ground electrode 104.
The optical waveguide 102 is of the Mach-Zehnder type formed from an incoming waveguide 102a, two parallel waveguides 102b and 102c, and an outgoing waveguide 102d. As shown in FIG. 14(b), the signal electrode 103 is arranged just above the parallel waveguide 102b in order to utilize a refractivity variation by an electric field in the Z-direction. The ground electrode 104 is formed in a predetermined spaced relationship from and along outer edges of the signal electrode 103 over the overall substrate face on which the optical waveguide 101 is formed.
It is to be noted that, in FIG. 14(b), reference numeral 105 denotes a buffer layer. The buffer layer 105 is layered between the substrate 101 and the signal electrode 103 and ground electrode 104 in order to prevent a phenomenon that light which propagates in the parallel waveguides 102b and 102c is absorbed by the signal electrode 103 and the ground electrode 104.
The optical modulator can be applied as such an optical modulator module 100 as, for example, shown in FIG. 15 to a practical communication system. In the optical modulator module 100 of FIG. 15, the substrate 101 is mounted on a housing 107. Inner walls 109 of the housing 107 partly extend inwardly from and are formed integrally with the housing 107 in such a manner as to uniformly sandwich side longitudinal faces with respect to a face of the substrate 101 on which the optical waveguide 102 is formed.
Further, reference numeral 108 denotes a connector connected to the signal electrode 103 for supplying an input signal from the outside to the signal electrode 103. Where the optical modulator 106 is driven at a high rate, one end of the signal electrode 103 as an output terminal and a terminal end of the ground electrode 104 are connected to each other through a resistor R to form a progressive wave electrode, and a microwave signal is applied from the other end of the signal electrode 103 serving as an input terminal. Thus, the refractivity of the optical waveguide 102 varies like +Δn and −Δn in response to an electric field variation by the microwave signal applied to the signal electrode 103, and thereupon, the phase difference between the parallel waveguides 102b and 102c varies. As a result, modulated signal light can be outputted from the outgoing waveguide 102d. 
However, in the conventional optical modulator module and optical modulator 100 having such a configuration as described above, if a high-frequency signal is applied to the progressive wave electrode, then a microwave having a certain frequency resonates in the substrate 101. Therefore, there is a subject that such a dip 110 as seen in FIG. 16 appears in a frequency characteristic (S21 characteristic) of the microwave which propagates in the signal electrode 103 and degrades the light response characteristic.
Conventionally, in order to suppress the dip, several methods have been proposed including a method wherein the thickness or the width of the substrate is reduced or a groove is formed on the rear face of the substrate (refer to Japanese Patent Laid-Open No. 241115/1993) and another method wherein the substrate is formed such that the shape of a cross section thereof varies in a longitudinal direction (refer to Japanese Patent Laid-Open No. 128623/1995). However, in the methods just described, since the substrate itself is worked, the mechanical strength thereof is reduced. Therefore, the methods have subjects to be solved in that handling of the substrate becomes difficult and that the long-term reliability is degraded.